1. Field of the Invention
The invention relates to a drive and evaluation unit, in particular for raising and lowering a side window in a motor vehicle.
2. Description of the Prior Art
For positional determination of displacement systems driven by commutator motors, it is known to evaluate the waviness of the armature current. This method is called the ripple count method. In it, a frequency that is proportional to the motor rpm and is modulated to the armature current is analyzed by means of an evaluation unit. The possible system-characteristic frequency that can be used for this purpose is caused by the commutation between the individual armature windings and thus makes it possible to subdivide one revolution of the electric motor into a number of partial revolutions corresponding to the number of commutations, so that for instance if there are six segments of the commutator, six maximum current waviness points per revolution of the armature occur. These maximum points are counted by the evaluation unit. Since a fixed relationship exists between the revolution of the armature of the commutator motor and the position of the displacement object, the position of the displacement object can be determined by way of evaluating the position of the commutator motor. In known drive and evaluation units, it is problematic that the amplitudes of the vibration modulated to the armature current usually have such a low intensity that miscounts and hence incorrect position determinations result.